Telomeres are specialized regions at the tips of linear eukaryotic chromosomes which are required for their integrity. The DNA near the ends of the chromosomal DNA of a number of single- celled eukaryotes has been isolated and characterized, but very little is known about the corresponding regions of the DNA from the chromosomes of metazoans. The major long-term objectives of the proposed research are to better understand: 1) the structure of the tips of the chromosomes of Drosophila melanogaster and how this structure is adapted for carrying out the functions of the telomere; 2) the consequences of mutational loss of the DNA from the chromosome tip region; 3) the effect of chromosomal location on gene expression, particularly for genes translocated to the telomeric regions. The experiments proposed in this application will focus on the DNA sequence organization at the tips of normal and rearranged Drosophila chromosomes. Three P-element transposons carrying the white gene, each inserted at the tip of a different chromosome, will be used to: 1) isolate surrounding telomeric DNA sequences using recombinant DNA techniques; 2) promote and analyze adjacent DNA rearrangements; 3) study the expression of genes ectopically placed in a telomeric environment. In preliminary studies one of these transposons has been induced to cause local chromosomal rearrangements at high frequency. Flies carrying such rearrangements are easily recognized because of the effect of the rearrangement on the expression of the white gene that is part of the transposon. Some of the rearrangements appear to be terminal chromosomal deletions in which the transposon is brought to the very end of the chromosome. Interestingly, over the course of subsequent generations of flies the transposon DNA sequences closest to the end of the chromosome are gradually lost.